1. Field of the Invention
The invention relates to protecting metal membrane seals in FCC regenerators from thermal stress, and to a catalytic cracking process using such regenerators.
2. Description of the Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-2900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
Similar advancements have been made in the FCC regenerator. Most new FCC units now use a high efficiency regenerator (H.E.R.), characterized by a fast fluidized bed coke combustor, a dilute phase transport riser mounted above the coke combustor, and a second dense bed, for collection of regenerated catalyst for recycle to the reactor and frequently for recycle to the coke combustor as well. Such regenerators are now widely used, because they allow FCC units to operate with roughly half the catalyst required when using a prior art, bubbling dense bed regenerator. These units do have some mechanical problems.
The H.E.R. design is generally "stacked", that is, the coke combustor comprises the base of the unit, the transport riser is mounted directly above the coke combustor, usually via a cone or dome shaped transition section, and the second dense bed of catalyst forms around the transport riser, usually over the cone shaped transition section.
The cone or dome shaped transition is subject to extreme thermal stress during operation of the FCC unit. The transition section must be strong, to support itself and a large, bubbling dense phase fluidized bed of catalyst. It must also be sealed, to isolate the fast fluidized bed region from the second dense bed region, and usually must be lined with a refractory material to withstand erosion.
The cone or dome shape is inherently strong, much stronger than a typical grid floor, because arches and domes are stronger than post and lintel construction. The cone or dome is also rigid, due partly to its shape and greatly to the refractory lining which must be added to withstand years of sandblasting in the FCC unit. This strength and rigidity makes it very difficult to reliably seal the cone or dome to the top of the coke combustor. The cone expands and contracts at a different rate than the other parts of the regenerator, and many times deformation and cracks develop which require shutdown and dismantling of the regenerator. This is expensive not only from the point of view of lost production, but also from the cost of repairs, which can and do cost millions of dollars, because of the large size of the vessels involved.
We knew there had to be a better way to seal a cone to a generally cylindrical pressure vessel than just welding the cone to the cylinder. We reviewed the technology available on sealing metal membranes in FCC units and found nothing that was directly applicable or completely satisfactory. Perhaps the closest was U.S. Pat. No. 4,493,816, Insulation System for Process Vessel, which is incorporated herein by reference. This patent recognized the problem of sealing a grid floor, lined with refractory material, to a process vessel wall in a manner such that expansion of the grid floor does not stress the refractory. The patent teaches a sealing system which can be used on vertical and horizontal surfaces, but which was not directly applicable to solving our problem, namely sealing surfaces which are neither vertical nor horizontal.
We developed a way to seal a cone or dome shaped surface to the top of an FCC regenerator vessel which minimizes thermal stress cracking of the cone or dome.